Some types of implantable medical devices, such as cardiac pacemakers or implantable cardioverter defibrillators, provide therapeutic electrical stimulation to a heart of a patient via electrodes of one or more implantable leads. The therapeutic electrical stimulation may be delivered to the heart in the form of pulses or shocks for pacing, cardioversion or defibrillation. In some cases, an implantable medical device (IMD) may sense intrinsic depolarizations of the heart, and control the delivery of therapeutic stimulation to the heart based on the sensing.
Cardiac resynchronization therapy (CRT) is one type of therapy delivered by an IMD. CRT may help enhance cardiac output by resynchronizing the electromechanical activity of the ventricles of the heart. Ventricular dysynchrony may occur in patients that suffer from congestive heart failure (CHF). In some examples, CRT involves delivery of pacing pulses to both ventricles (“biventricular pacing”) to synchronize their contraction. In other examples, CRT involves delivery of pacing pulses to one ventricle to synchronize its contraction with that of the other ventricle, such as pacing the left ventricle to synchronize its contraction with that of the right ventricle.
In general, it is believed that the benefit of CRT depends on consistent delivery of CRT, e.g., depends on CRT being delivered essentially constantly. Some existing IMDs track the amount of time CRT was or was not delivered. Based on such data, an external programmer in communication with the IMD may provide a value of a diagnostic metric to a user. An example of a diagnostic metric for evaluating CRT is the percent of a period time, e.g., since the last interrogation of the implantable medical device, that the IMD has delivered (or not delivered) CRT to the user. This metric has been referred to as % CRT.